There is an increased interest in the application of control to combustion. An objective is to optimize combustor operation, monitor the process and avoid instabilities of the flame and their severe consequences. An objective is to improve the system performance, for example, by reducing the levels of harmful emissions, and to extend the stability domain by reducing oscillations induced by coupling between resonance modes and combustion. Correspondingly, monitoring of the flame by means of sensors in addition to the monitoring of the combustion products and their composition is becoming increasingly important.
The corresponding flame detection or flame scanning devices should be as reliable as possible, should allow the determination of as many as possible parameters of the flame, should be as broadly applicable as possible, and should be resistant to typical temperatures around the flames. All these requirements in principle might be met by using standard techniques. However, the problem arises that the more technically sophisticated the method of detection, and the more reliable the chosen technology, the more expensive the device becomes. Correspondingly, therefore, there is a demand for simple but nevertheless very sensitive, broadly applicable and reliable devices at low cost.
A flame scanner or flame detectors may be passive devices which record light emissions within the combustion chamber, such as IR emission of particulates following the Planck law for a given temperature, emission of different molecular species which are present during the heat release process like OH*, CH*, C2*, etc.
Other devices record the presence of molecules in IR by applying absorption spectroscopy. Such devices need a light source, a dispersive element, and an IR detector. These devices are active since they need a light source.
The main flame scanners give the flame on/off-status or eventually the frequency of the flame fluctuation.
More advanced sensors may give the following information:
flame parameter detection like λ or φ (air/fuel or fuel/air ratio); OH/CH, CH/CN, OH/C2, C2/CH ratios give information about the temperature or stochiometry;
temperature via measurement with 2- or 3-colors pyrometry, via measurement with H2O & CO(CO2) absorption in the MIR and NIR (tuneable lasers) range;
imaging: CMOS, CCD camera multi-bands detection, flame on/off-detection;
UV and IR measurements (UV, OH, CH, C2 chemiluminescence; VIS/IR Planck radiation of soot particulates);
UV/IR detection and heat release fluctuation thresholds method (high or low frequency changes).
Corresponding devices which can be used industrially are known.
For example, EP 0 616 200 discloses a device in which a camera that photographs the flame includes a plurality of photosensors which are integrated into the camera and are disposed on an imaging face thereof. The camera provides a flame image which can be displayed, and the corresponding images are analyzed for the derivation of combustion properties of the flame. The photosensors constitute a photosensor group in which each of the photosensors has a detection wavelength range, and the group of sensors covers the full contiguous visible radiation range. The photosensor values are used for the detection of radicals such as, for example, CH, OH and the like, the chemiluminescence of which can be detected in the visible range. An objective behind the device is to have a combined camera/spectrum detection device, wherein the former allows flame shape detection and the like, and the latter covers detection of the full visible wavelength range.
U.S. Pat. No. 6,045,353 discloses an a device for controlling the combustion of a burner. The device includes means for viewing the radiation emitted by the flame for collecting frame radiation intensity data as a function of time. The radiation is transported to an optical processor in which specific spectral regions of radiation are converted into electrical signals, which are then processed by a signal processor for integrating flame radiation intensity for the specific spectral regions of a period of time. The output of the signal processor is subsequently used to control the oxidant flow, fuel flow or both. Specifically, the device is located in the refractory block of the burner.
U.S. Pat. No. 6,318,891 discloses a device for determining the adiabatic temperature of a flame. The device comprises a sensor fiber which is coupled into a spectrograph. In the spectrograph, an adjustable section of the spectrum is acquired with a high resolution for individual radicals. The spectrograph thus includes a dispersive element, and the selected ranges are subsequently used in combination with theoretically calculated emission spectra for the determination of the Boltzmann temperature, which is then correlated with the adiabatic temperature of the flame.
WO 2006/091617 discloses a device for monitoring the flame across a contiguous spectrum by means of a plurality of discrete ranges measured by photodiodes. A beam splitter, which in this case is a dispersive element, is used for directing the collected light onto each of a multitude of photodiodes which cover the contiguous spectrum. The corresponding spectral range which is fully covered extends from 300 nm-1100 nm,